梯度结构铜及铜合金的塑性变形机理及力学行为研究

发布时间：2018-04-12 13:38

本文选题：梯度结构 + 表面机械研磨处理　；参考：《昆明理工大学》2017年硕士论文

【摘要】：梯度结构材料在一定程度上较好地解决了传统材料的强度-塑性“倒置”(trade-off)关系,不仅呈现出优异的高强度高塑性匹配,还具备良好的表面耐磨性、抗疲劳等性能,因此也越来越受到更多关注。然而,目前有关梯度结构的可控制备、梯度结构材料的组织性能关系及变形力学行为等研究还不够系统,仍然存在诸多问题。本文采用表面机械研磨处理(Surface mechanical attrition treatment,SMAT)方法在纯铜及不同层错能的铜铝合金中获得梯度结构,梯度结构的存在有效提高了材料强度,同时其塑性得到较好保持。对纯铜分别在不同时间和温度下进行SMAT处理,SMAT处理时间越长、处理温度越低得到的梯度结构层更厚。温度降低动态回复被抑制,晶粒细化更明显,变形方式逐渐由位错转变为孪生,低温处理的样品中有大量孪晶出现,屈服强度高。经SMAT处理得到的梯度结构纯铜,当其梯度层所占体积分数达到一定程度后,加工硬化率在低应变范围内存在一个缓慢上升的阶段,出现加工硬化率反转(up-turn)现象,形成明显的额外加工硬化。此现象与拉伸变形过程中的可动位错运动有关,相对可动位错密度在拉伸过程中呈现先下降后上升的趋势。将不同层错能的铜铝合金(Cu-2.2wt.%Al、Cu-4.5wt.%Al、Cu-6.9wt.%Al)在低温下进行SMAT处理获得梯度组织,发现具有中等层错能的Cu-4.5wt.%Al合金更适合于采用SMAT工艺来实现最佳的强度塑性配合。其屈服强度的提升主要由梯度(Gradient structure,GS)层贡献,均匀延伸率则受变形过程中的动态回复控制,根据Kocks-Mecking模型拟合得出,中等层错能的Cu-4.5wt.%Al样品中代表动态回复/位错湮灭的K2值最小。应力松弛实验显示,Cu-4.5wt.%Al样品的可动位错湮灭速率最慢,即位错回复受到阻碍,可以推迟颈缩的产生,使样品保持良好塑性。XRD结果表明,样品在变形过程中产生的高密度孪晶可有效地降低可动位错损耗,同时能够在一定程度上使样品保持较高的加工硬化能力。在梯度结构材料中,根据Hall-Patch公式由简单混合法则计算出的样品屈服强度远小于实际样品的屈服强度。梯度结构的高强度并非完全由晶粒细化贡献,还由在梯度变化的晶粒中大量堆积的位错产生的背应力来提供。梯度结构样品的包辛格效应比均匀样品更明显,应变梯度越大,则产生的几何必须位错(Geometrically necessary dislocations,GNDs)越多,阻碍新的位错产生和增殖,因此背应力也更高,对屈服强度的贡献更强,强度得以明显提升。梯度结构材料主要依靠晶粒细化及梯度应变产生的GNDs来实现协同强化。
[Abstract]:To some extent, gradient structural materials have solved the "inversion" relationship between strength and plasticity of traditional materials, showing not only excellent high strength and high plasticity matching, but also good surface wear resistance, fatigue resistance and so on.As a result, more and more attention has been paid to it.However, the studies on the controllable preparation of gradient structure, the relationship between microstructure and properties and deformation mechanical behavior of gradient structure materials are still not systematic, and there are still many problems.In this paper, the surface mechanical attrition treatment-Smatt method is used to obtain gradient structure in pure copper and copper aluminum alloy with different stacking fault energy. The existence of gradient structure improves the strength of the material and keeps the plasticity of the alloy.The longer the SMAT treatment time of pure copper is, the thicker the gradient structure layer is when the treatment temperature is lower.The decrease of temperature dynamic recovery was restrained, the grain refinement was more obvious, the deformation mode gradually changed from dislocation to twinning, a large number of twins appeared in the samples treated at low temperature, and the yield strength was high.When the volume fraction of gradient layer reached a certain degree, the work hardening rate of pure copper with gradient structure obtained by SMAT treatment increased slowly in the range of low strain, and the work hardening rate reversed up-turn phenomenon.The formation of obvious additional work hardening.This phenomenon is related to the movable dislocation movement during tensile deformation, and the relative movable dislocation density decreases at first and then increases during tensile deformation.Cu-2.2wt.Alncu-4.5wt.Alanum Cu-6.9wt.Al) with different stacking fault energy was treated with SMAT at low temperature to obtain gradient microstructure. It was found that the Cu-4.5wt.%Al alloy with medium stacking fault energy was more suitable to achieve the best strength and plastic fit by using the SMAT process.The increase of yield strength is mainly contributed by gradient structure GSlayer, and the uniform elongation is controlled by dynamic recovery during deformation. According to Kocks-Mecking model fitting, the K _ 2 value representing dynamic recovery / dislocation annihilation is the smallest in Cu-4.5wt.%Al samples with medium fault energy.The stress relaxation experiment shows that the dislocation annihilation rate of Cu-4.5wt.Al sample is the slowest, and the recovery of the spot fault is blocked, which can delay the occurrence of necking shrinkage and keep the sample in good plasticity. XRD results show that the dislocation annihilation rate of Cu-4.5wt. Al sample is the slowest.The high density twin produced in the process of deformation can effectively reduce the loss of movable dislocations, and at the same time, it can make the samples maintain high working-hardening ability to a certain extent.In gradient structural materials, the yield strength of the sample calculated by the simple mixing rule based on the Hall-Patch formula is much smaller than that of the actual sample.The high strength of gradient structure is not only contributed by grain refinement, but also by the back stress caused by dislocation accumulated in gradients.The Bauschinger effect of gradient structure samples is more obvious than that of homogeneous samples. The bigger the strain gradient is, the more geometric geometry must be generated, which hinders the generation and proliferation of new dislocations, so the back stress is higher and the contribution to yield strength is stronger.The strength was significantly increased.Gradient structure materials mainly rely on grain refinement and GNDs generated by gradient strain to achieve synergistic strengthening.
【学位授予单位】：昆明理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG146.11

【参考文献】